The reforming of petroleum hydrocarbon streams is an important petroleum refining process that is employed to provide high octane hydrocarbon blending components for gasoline. The process is usually practiced on a straight run naphtha fraction that has been hydrodesulfurized. Straight run naphtha is typically highly paraffinic in nature, but may contain significant amounts of naphthenes and minor amounts of aromatics or olefins. In a typical reforming process, the reactions include dehydrogenation, isomerization, and hydrocracking. The dehydrogenation reactions typically will be the dehydroisomerization of alkylcyclopentanes to aromatics, the dehydrogenation of paraffins to olefins, the dehydrogenation of cyclohexanes to aromatics, and the dehydrocyclization of paraffins to aromatics. The aromatization of the n-paraffins to aromatics is generally considered to be the most important because of the high octane of the resulting aromatic product compared to the low octane ratings for n-paraffins. The isomerization reactions include isomerization of n-paraffins to isoparaffins, and the isomerization of substituted aromatics. The hydrocracking reactions include the hydrocracking of paraffins and hydrodesulfurization of any sulfur that is remaining in the feedstock.
It is well known in the art that several catalysts are capable of reforming petroleum naphthas and hydrocarbons that boil in the gasoline boiling range. Examples of known catalysts useful for reforming include platinum and optionally rhenium or iridium on an alumina support, platinum on zeolite X and zeolite Y, platinum on intermediate pore size zeolites as described in U.S. Pat. No. 4,347,394, and platinum on cation exchanged zeolite L. U.S. Pat. No. 4,104,320 discloses the dehydrocyclization of aliphatic hydrocarbon to aromatics by contact with a catalyst comprising a zeolite L containing alkali metal ions and a Group VIII metal such as platinum.
The conventional reforming catalyst is a bifunctional catalyst that contains a metal hydrogenation-dehydrogenation component, which is usually dispersed on the surface of a porous inorganic oxide support, usually alumina. Platinum has been widely used commercially in the production of reforming catalysts, and platinum on alumina catalysts have been commercially employed in refineries for the past few decades. More recently, additional metallic components have been added to the platinum to further promote the activity or selectivity, or both. Examples of such metallic components are iridium, rhenium, tin and the like. Some catalysts possess superior activity, or selectivity, or both as contrasted with other catalysts. Platinum-rhenium catalysts, for example, possess high selectivity in comparison to platinum catalysts. Selectivity is generally defined as the ability of the catalyst to produce high yields of desirable products with concurrent low production of undesirable products, such as gaseous hydrocarbons.
It is desirable to maximize xylene and benzene production and ultimately para-xylene and benzene production. The problem of how to do this has not been previously solved. The prior art has dealt with the problem of maximizing only benzene production when processing a wide boiling C.sub.5 -C.sub.11 naphtha but has not addressed how to maximize first para-xylene production and secondly benzene production. Note that maximizing benzene production should not occur by downgrading C.sub.8 and C.sub.9 aromatics to benzene. This is especially important as para-xylene has historically commanded a premium above benzene.
There exist several processes for dividing naphtha feedstreams into a higher boiling cut and a lower boiling cut and reforming these cuts separately. U.S. Pat. No. 2,867,576 discloses separating straight run naphtha into lower and higher boiling cuts, in which the higher boiling cuts are reformed with a hydrogenation-dehydrogenation catalyst with the liquid reformate produced being routed to an aromatics separation process. The paraffinic fraction obtained from the separation process is blended with the lower boiling naphtha fraction and the resulting blend is reformed with a reforming catalyst, which may or may not be the same type employed in reforming the high boiling cut.
U.S. Pat. No. 2,944,959 discloses fractionating a full straight run gasoline into a light paraffinic fraction, C.sub.5 and C.sub.6, that is hydroisomerized with hydrogen and a platinum-alumina catalyst, a middle fraction that is catalytically reformed with hydrogen and a platinum-alumina catalyst, and a heavy fraction that is catalytically reformed with a molybdenum oxide catalyst and recovering the liquid products. U.S. Pat. Nos. 3,003,949, 3,018,244 and 3,776,949 also disclose fractionating a feed into a C.sub.5 and C.sub.6 fraction, that is isomerized, and a heavier fraction that is reformed.
Other processes for dividing feedstocks and separately treating them include: U.S. Pat. Nos. 3,172,841 and 3,409,540 disclose separating fraction of a hydrocarbon feedstock and catalytically reforming various fractions of the feed; U.S. Pat. No. 4,167,472 discloses separating straight chain from non-straight chain C.sub.6 -C.sub.10 hydrocarbons and separately converting to aromatics; and U.S. Pat. No. 4,358,364 discloses catalytically reforming a C.sub.6 fraction and producing additional benzene by hydrogasifying a C.sub.5- fraction, a fraction with a boiling point above 300.degree. F. and the gas stream produced from catalytic reforming.
U.S. Pat. No. 3,753,891 discloses fractionating a straight run naphtha into a light naphtha fraction containing the C.sub.6 and a substantial portion of the C.sub.7 hydrocarbons and a heavy naphtha fraction boiling from about 200.degree. to 400.degree. F.; then reforming the light fraction to convert naphthenes to aromatics over a platinum-alumina catalyst or a bimetallic reforming catalyst; separately reforming the heavy faction, then upgrading the reformer effluent of the low boiling fraction over a ZSM-5 type zeolite catalyst to crack the paraffins and recovering an effluent with improved octane rating.
U.S. Pat. No. 4,645,586 discloses parallel reforming of a hydrocarbons feed. In one stream, the hydrocarbons are reformed with an acidic catalyst. In the second stream, the hydrocarbons are reformed with a non-acidic catalyst. That patent is silent as to the composition of each fraction. Preferably, the acidic bi-functional reforming catalyst is not presulfided.
U.S. Pat. No. 4,897,177 discloses using a monofunctional catalyst to reform a hydrocarbon fraction having less than 10% by volume of C.sub.9+ hydrocarbons. This fraction is either a C.sub.6, C.sub.7, C.sub.8, C.sub.6 -C.sub.7, C.sub.7 -C.sub.8, or C.sub.6 -C.sub.8 fraction, with the most preferred being a C.sub.6 -C.sub.8 fraction. That fraction can contain up to 15 vol. % hydrocarbons outside the named range (col. 3, line 44-49). A heavier fraction can be reformed using a bifunctional catalyst on an acidic metal oxide. That bifunctional catalyst can be a Pt/Sn/alumina catalyst.
U.S. Reissue Patent No. 33,323 discloses solvent extraction of a light fraction of a reformate. The goal of that patent is to maximize benzene production only. A hydrocarbon feed is separated into a lighter fraction (a C.sub.6 cut that contains 15-35 lv % C.sub.7+) and a heavier fraction (all remaining C.sub.7 and heavier components). The lighter fraction is reformed in the presence of a non-acidic catalyst to maximize benzene yield. The heavier fraction is reformed in the presence of an acidic catalyst. The reformate from the non-acidic catalyst is introduced into an extraction where an aromatic extract stream and a non-aromatic raffinate stream are recovered. The raffinate stream can be recycled to the feed.
The paper entitled "New Options For Aromatics Production" presented to the 20th Annual 1995 Dewitt Petrochemical Review (Houston, Tex., Mar. 21-23, 1995) by J. D. Swift et al. related recent improvements in UOP's process for the production of benzene and para-xylene. Case studies were presented to demonstrate the benefits of using that process to increase total aromatics production from a fixed quantity of naphtha. One configuration of that process involved a split-feed process, but it is unclear what the composition of each feed was.